1. Field of the Invention
This invention relates to the field of control systems for industrial robots and, more particularly, pertains to control systems that guide the movement of a tool held by a robot arm along a predetermined path.
2. Background of the Invention
In the wrist mechanism of an industrial robot, among all possible combinations of mechanisms having as many as three joints whose axes are mutually perpendicular or parallel, only those whose consecutive joints are mutually perpendicular can produce as many rotational degrees of freedom as the number of joints in the mechanism. However, three-axis wrist mechanisms have certain mechanical limitations, kinematic anomolies called by "gimbal lock", which deteriorate the performance of the wrist is a region within the working envelope. When the third axis of the wrist becomes parallel to the first axis, gimbal lock occurs. This condition causes the wrist mechanism to loose one degree of freedom. To overcome these limitations, some robots use either oblique chain or compound chain linkages, wherein the articulation occurs other than with rotary or sliding joints or by connecting the links to two or more other links.
An effective design that eliminates wrist anomolies divides the total pitch and yaw articulations into several smaller parts. A wrist employing this technique uses more than three rotary joints suitably constrained to result in no more than three overall degrees of freedom. The articulation of individual joints is reduced, although the range of overall articulation may be increased.
Three-axis robot wrist mechanisms exhibit singularities which restrict their mobility at certain orientations of the end-effector or tool carried by the robot arm. Near singularities, the rotational speeds about certain axes of the wrist mechanism rise to very high speeds to achieve the desired motion of the tool. Often the required speed of rotation about the wrist mechanism axes exceeds the design limits. The speed of the tool may have to be reduced greatly until it passes the region of the singularity, but this is generally unacceptable because either the application requires smooth continuous motion or the period for the operation performed by the robot becomes too long.